The field of elongate soil sensors includes many types of soil sensors but the shape of the housing that is inserted into the ground will largely be the same. The soil sensors are enclosed in housings, some examples of housings are disclosed in the abovementioned patents but in the main the housings are elongate to permit multiple sensors to be located along the internal length of the elongate housing, at various depths in the soil or material to be sensed. Of particular note is that the housings are all cylindrical and have a constant diameter along the full inserted length. The outer shape of the hollow elongate sensor housing is largely dictated by the shape of the active sensor element which is an element in a tuned circuit which is also part of a more complex moisture/complex dielectric constant sensor, which can if used appropriately, and as is described in detail in U.S. Pat. No. 5,418,466, provide salinity measurement or trend indications of the moisture of the soil within which the housing is inserted at the various depths of the sensor elements part of the sensor circuit.
In many soil sensors capacitive active elements are used as part of a tuned circuit and generally a capacitive sensor element is comprised of two metal plates physically separated at a fixed distance but located and arranged so that the electromagnetic field they generate and the effects of the field of influence of that generated field is maximally projected into and received from the surrounding soil, the characteristics of which are then measureable by way of analysis of the signal associated with the tuned circuit. In the soil moisture sensors disclosed in the referenced patent specifications, the plates are formed by metallic rings, typically having a height much greater than their thickness and a diameter that will determine or be determined by the frequency of operation of the tuned circuit. The sensor rings are in the main positioned so that the circular outer face of each ring of the pair of rings is close to the inner surface of the generally cylindrical housing, which in effect is close to the surrounding soil which is to be sensed by those sensors when in operation. Multiple pairs of rings are typically spaced 10 cm apart along the internal length of the housing to provide measurements of the soil characteristics at different depths in the ground.
The referenced sensor arrays are housed in elongate cylindrically shaped housings and the reasons for carefully creating a cylindrical opening in the ground, include the following:                a. To ensure that the ground in the immediate vicinity of the hollow elongate sensor housing and hence the sensors therein remains as undisturbed as possible.        b. That the physical fitment between the hollow elongate sensor housing and the surrounding ground is such that there is no gap or gaps between the surrounding soil and the outer surface of the elongate sensor housing along substantially its full length. Gaps, if they existed, would create the potential for the creation of preferential water seepage channels from the surface of the soil and in the immediate vicinity of the sensor, which will skew or make un-useable the readings of soil moisture and other characteristics of the surrounding soil detected by the sensor/s within the hollow elongate sensor housing. Further, there can be shadowing if the housing extends above the ground level that can also affect the moisture distribution in the immediate vicinity of the sensor.        
Some of these characteristics do not become an issue if the gap or gaps do not exist but to achieve that outcome using the prior arrangements there is a need to ensure that the inner wall of the opening created by the auger is relatively smooth and of constant inner diameter along its full depth. This has been the intention of the techniques described in at least one of the referenced documents, in one example, by slightly under-sizing the opening created by the auger relative to the constant outer diameter of the inserted hollow elongate sensor housing, and cutting or slicing away a portion of the inner wall of the prepared opening with the inserted end of the hollow elongate sensor housing adapted to do that slicing as it is inserted into the prepared opening.
It will be noted in the referenced documents that the elimination of a gap or gaps between the hollow elongate sensor housing and surrounding soil is identified as a requirement for the proper operation of the in-situ soil sensor. However, the installation process described in those patents where constant diameter cylindrical hollow elongate sensor housings are used does not ensure that outcome every time. One problem encountered includes, that any wobbling of the auger by the operator during the creation of the opening can create larger excursions from the sheared volume created when the hollow elongate sensor housing with a cutting arrangement is inserted, such that gaps are left at one or more locations along the length of the inserted hollow elongate sensor housing. The gap, if air filled is substantial enough or if the gap is occupied by water, affects the sensor measurements in the gap regions and they will not be indicative of the soil characteristics in the field of influence of the sensor near or at those gap regions. Yet further, the type of soil can make a huge difference to how easy or hard it is to create a prepared opening because of the hardness, composition or even the current soil moisture content. It being known that if the soil is primarily clay then there will be considerable work in creating the prepared opening and sometimes as much work again inserting the hollow elongate sensor housing.